If a drug is microencapsulated and administered in vivo, the metabolism of the drug in vivo is suppressed, and the medicinal effect can be maintained over a long period of time. Because of such a merit, various microencapsulation technologies for active ingredients have been searched in the fields of drugs, cosmetics, food, etc. As one of these technologies, the so-called vesicles are attracting attention. The vesicles are small enclosed sacs having a bimolecular membrane structure and formed of amphiphilic substances. Because of the unique structure, the containment of active ingredients is possible, and they are attracting attention as the carrier in drug delivery system.
Numerous amphiphilic substances that form vesicles have been investigated. In particular, vesicles consisting of phospholipids derived from biological sources are called liposomes, and various studies have been carried out from the viewpoint of safety to the body. For example, there is a report that a vesicle dispersion was formed by combining a phospholipid and a specific cationic surfactant, and this was applied to cosmetics (for example, refer to Japanese Unexamined Patent Publication No. 2006-193461). However, the liposome components are mainly derived from natural sources; therefore, they are affected by pH and temperature as well as electrolytes. Thus, there are numerous restrictions in terms of stability over time.
There are also numerous reports of synthetic surfactants as other amphiphilic substances that form vesicles. In Japanese Unexamined Patent Publication No. 2006-104142, for example, the technology wherein a mono(long-chain aliphatic)tri(short-chain alkyl)ammonium salt, a di(long-chain aliphatic)di(short-chain alkyl)ammonium salt, and a higher alcohol are used has been disclosed. In recent years, vesicle compositions wherein multi-chain multi-hydrophilic group type surfactants are used have been investigated (refer to Japanese unexamined Patent Publication No. 2006-290894).
However, the technology for hair cosmetics in Japanese Unexamined Patent Publication No. 2006-104142 was sometimes unfavorable in terms of skin safety depending on the blending quantity of the cationic surfactant. In the technology in Japanese unexamined Patent Publication No. 2006-290894, an amino acid-based surfactant was used. Thus, the vesicles can be obtained by a simple method; however, the stability of the vesicles over time was not satisfactory.
In particular, the vesicles wherein amphiphilic diblock and multiblock copolymers, in which at least one block is hydrophobic and at least one block is hydrophilic, are used as the amphiphilic substance are called “polymersomes”. Their excellent mechanical stability and unique chemical properties, compared with the conventional liposomes and micelles, have been attracting attention (for example, refer to Science, 2002, 297, 967-973 and Proceedings of the National Academy of Sciences of the United States of America (PNAS), 2005, 102, 2922-2927).
Polymersomes can be stably prepared by numerous technologies common to those of liposomes (for example, refer to E. Biotechnol. Bioeng., 2001, 73, 135-145). Moreover, many-micron giant vesicles and monodisperse vesicles with a diameter of 100 nanometers can be obtained by film rehydration, sonication, and extrusion.
Polymersomes are also known to have a capability not only to entrap water-soluble hydrophilic compounds (drugs, vitamins, fluorophores, etc.) inside of their aqueous cavities but also to entrap hydrophobic molecules within their thick lamellar membranes. In addition, the size, membrane thickness, and the stabilities of those synthetic vesicles can be rationally adjusted by selecting a chemical structure of the block copolymer, number average molecular weight, hydrophilic to hydrophobic volume fraction, and various intermediary preparation methods. Accordingly, polymersomes can be provided with numerous attractive characteristics that are expected to be applied to various uses in medical imaging, drug delivery, and cosmetic devices (for example, refer to Journal of Controlled Release, 2005, 101, 187-198).
Specific vesicles of PEO-PEE (polyethylene oxide-polyethylethylene) or PEO-PBD (polyethylene oxide-polybutadiene) are known to form polymersomes with a membrane thickness of improved stability (for example, about 100 nm) compared with liposomes. For example, the PEO-PEE diblock copolymer introduced by Hillmeyer and Bates (refer to Macromolecules, 1996, 29, 6994-7002), especially EO 40-EE 37 (designated OE7, where EO is an ethylene oxide monomer and EE is an ethylethylene monomer), is reported to form a very thick membrane and show higher mechanical stability compared with any natural lipid membrane (refer to Science, 1999, 284, 1143-1164).